1. Field of the Invention
The present invention relates to a substrate accommodating tray used for transporting square or rectangular substrates, such as a glass substrate for a display or the like. The glass substrate is used for producing a display panel of, for example, a liquid crystal display device.
2. Description of the Related Art
A display panel for a liquid crystal display device usually includes a pair of display glass substrates opposed to each other and sealed together, and a liquid crystal material sealed between the pair of glass substrates. In order to produce such a display panel, glass substrates are transported to a display panel production plant. For transporting the glass substrates, a glass substrate accommodating box for accommodating a plurality of glass substrates is usually used. Glass substrates are used in display panels of various types of display devices as well as liquid crystal display devices. The above-mentioned type of glass substrate accommodating box is also used for transporting glass substrates used for various types of display devices other than liquid crystal display devices.
For producing display panels, the same type of glass substrate accommodating box is used for transporting display panels as half-finished products having electrodes on the glass substrate.
Recently, glass substrates having a thickness of 0.7 mm or less are widely used for various types of display panels. As the glass substrates are increasing in size, the planar area of the glass substrates which are carried to the display panel production plants is increasing, and rectangular glass substrates having a side length of 1.3 m or more are used.
Such a large and thin glass substrate easily warps. When a plurality of such glass substrates are vertically accommodated in the box with an interval, the glass substrates may warp and contact each other and break during transportation. In order to avoid this, it is necessary to keep an appropriate distance between the glass substrates in the box.
For example, a glass substrate having a thickness of 0.7 mm and a side length of 1.3 m or more may warp by 90 mm or more at the center thereof, when supported vertically along the periphery thereof with a support member having a width of 20 to 30 mm. Therefore, in a glass substrate accommodating box, it is necessary to keep a distance of at least 100 mm or more in the horizontal direction between the glass substrates in the box.
A glass substrate is usually removed from a glass substrate accommodating box using a glass substrate adsorption hand having a pair of flat adsorption pads. When such a glass substrate adsorption hand is used, it is necessary to insert each adsorption pad between two adjacent glass substrates and thus a sufficient space for inserting is necessary. A flat adsorption pad usually has a thickness of about 20 mm. Therefore, it is necessary for the distance between the glass substrates in the box to be the sum of a sufficient distance for preventing the glass substrates from contacting each other even when the glass substrates warp. Further, a distance of about 20 mm is also necessary for inserting the adsorption pad.
Since a plurality of glass substrates accommodated in a glass substrate accommodating box needs to have the suitable space between the substrates, the number of glass substrates which can be accommodated in a glass substrate accommodating box having a prescribed size is limited. Therefore, the space efficiency for transportation and storage, i.e., the number of glass substrates which can be accommodated per unit volume, decreases.
A glass substrate having a side length of 1.3 m or more is heavy having a weight per one substrate of about 5 kg. When the number of glass substrates accommodated in a glass substrate accommodating box is 20 or more, one operator cannot carry these glass substrates.
In order to solve these problems, Japanese Laid-Open Publication No. 10-287382 discloses a substrate tray cassette for accommodating one glass substrate. A substrate accommodating section of the substrate tray cassette has a lattice structure. The substrate tray cassette has an insertable engagement structure, in which a plurality of substrate tray cassettes can be stacked vertically. Such a substrate tray cassette allows a large and thin glass substrate to be accommodated without warping and thus without breaking during transportation. Since a higher number of substrate tray cassettes can be stacked vertically for transportation and storage, the space efficiency can be improved.
In the substrate accommodating tray in Japanese Laid-Open Publication No. 10-287382, the accommodated glass substrate is supported by support pins and each adsorption pad of a glass substrate adsorption hand is inserted in the space below this glass substrate. Such a space for the adsorption pads increases the size of the substrate accommodating tray. Further, since this substrate accommodating section is formed in a lattice shape, the strength of the tray has a limit and thus the number of substrate trays stacked vertically is limited.
In order to solve these problems, a substrate accommodating tray for mounting a glass substrate on a flat support member without warping is considered. However, when the glass substrate is mounted on the support member without warping, the support member contacts with a large area of the glass substrate, and thus the glass substrate may break during transportation depending on the characteristic of the support member.
For preventing the glass substrate from breaking, a synthetic resin foam body may be used as the support member. The synthetic resin foam body has comparatively excellent elasticity. However, even when the support member is made with the synthetic resin foam body, there are the following problems. That is, the surface of the glass substrate is damaged by the support member depending on the characteristic thereof. Further, a dust is generated due to the friction with the glass substrate and adheres to the glass substrate.
For preventing the dust from adhering to the glass substrate, it is considered that an antistatic function is added to the support member. For example, Japanese Laid-Open Publication No.2003-251769 discloses a foam laminated sheet of an antistatic polypropylene resin. This foam laminated sheet of an antistatic polypropylene resin is used as a package for a heavy object, for example, a large-sized liquid crystal panel.
In this foam sheet, an unfoamed polypropylene resin layer incorporated with an antistatic agent is formed at least on one side of a foam polypropylene resin layer, and this foam sheet is suitably used as a packaging material of a liquid crystal panel. However, such a foam laminated sheet of an antistatic polypropylene resin has insufficient elasticity and high rigidity, i.e., high hardness, and thus damages the glass substrate or generates dust due to the friction with the glass substrate. In the unfoamed polypropylene resin layer incorporated with an antistatic agent, the antistatic agent is easily released due to the friction with the glass substrate, and thus a charging function is not stabilized for a long period of time. Furthermore, since the unfoamed polypropylene resin layer has a laminated structure of a foam layer and an unfoamed layer, it cannot be easily produced.
A molded object is produced by mixing an antistatic agent such as carbon or the like to a synthetic resin beads as a raw material and foaming it. This molded object is broken by an impact when supporting a heavy object such as a glass substrate, since brittleness on the foam board surface increases, i.e., the surface becomes weak. (Referring to a paragraph of [0006] in Japanese Laid-Open Publication No.2003-251769)
Furthermore, when an antistatic agent having conductivity is mixed with a resin being an organic substance in order to give antistatic properties to the foam sheet, the resin on a foam sheet surface contacts with the surface of the glass substrate and is transferred on it. Under the condition where temperature and humidity are fixed, for example, in a clean room, the resin is hardly transferred on the glass substrate and is not at a visible level. Therefore, it is not a big problem in the liquid crystal panel having electrodes on the glass substrate, or the like. However, under the condition where the temperature is about 70° C. and humidity is about 80%, for example, during the transportation by a fruck in summer, if the resin is transferred onto the glass substrate before use for a liquid crystal panel, the processing yield for a fine process to make the liquid crystal panel decreases.
The present invention solves these problems and has an object to provide a substrate accommodating tray for effectively transporting and storing a substrate, such as a display glass substrate or the like. This substrate accommodating tray can effectively transport and store the substrate without dust adhering to the substrate and without breaking the substrate by providing an antistatic friction. This substrate accommodating tray has not a problem generated to have the antistatic friction, that is, the resin of the tray is transferred on the substrate.